1. Field of the Invention
The present invention relates to an ESD protection device, in particular, to an ESD protection device such as a single component having an ESD protection function only (ESD protection device) or a composite component (module) having an ESD protection function and another function.
2. Description of the Related Art
Electro-static discharge (ESD) is a phenomenon that, when a charged electrically conductive object (such as a human body) comes into contact with or comes sufficiently close to another electrically conductive object (such as an electronic device), a strong discharge is generated. ESD causes problems such as damage to and malfunction of electronic devices. To avoid such problems, an excessively high voltage generated at the time of discharge needs to be prevented from being applied to circuits of electronic devices. For such a purpose, ESD protection devices are used and they are also called surge absorbing devices or surge absorbers.
For example, such an ESD protection device is disposed between a signal path of a circuit and the ground. The ESD protection device has a structure in which a pair of discharge electrodes are opposed to each other at a distance from each other. Accordingly, the device has a high resistance and hence signals do not flow to the ground in the normal usage state. However, when an excessively high voltage is applied, for example, in the case of application of static electricity to a cellular phone through the antenna of the cellular phone, a discharge is generated between the discharge electrodes of the ESD protection device so that the static electricity can be made to flow to the ground. As a result, the voltage due to static electricity is not applied to the circuit disposed downstream of the ESD device to thereby protect the circuit.
For example, an ESD protection device illustrated in FIG. 16 (exploded perspective view) and FIG. 17 (sectional view) has the following configuration. In a ceramic multilayer substrate 7 in which insulating ceramic sheets 2 are laminated, a hollow portion 5 is formed; discharge electrodes 6 that are in electrical connection with outer electrodes 1 are disposed in the hollow portion 5 so as to oppose each other; and a discharge gas is contained in the hollow portion 5. When a voltage that produces an electrical breakdown is applied between the discharge electrodes 6, a discharge is generated between the discharge electrodes 6 in the hollow portion 5. As a result of this discharge, the excessively high voltage is introduced to the ground. Thus, the circuit disposed downstream of the ESD protection device can be protected (See, for example, Japanese Unexamined Patent Application Publication No. 2001-43954).
In such an ESD protection device, the ESD responsivity needs to be adjusted by changing the area of the opposing regions of the discharge electrodes. However, this adjustment is limited by, for example, the product size. Accordingly, desired ESD responsivity is less likely to be achieved.
In addition, the ESD protection device has the following problems. When static electricity at a high voltage is successively applied to the device, the discharge electrodes begin to melt. Thus, a short circuit is caused between the discharge electrodes; or the distance between the discharge electrodes is increased and the discharge starting voltage is increased.